In the mining industry, portable power cables, referred to as trailing cables, are used to supply power to equipment such as pumps, drills, shovels, and mining machines. A trailing cable includes three phase conductors, a ground conductor (also referred to as a ground wire), and an insulated ground-check wire (sometimes referred to as a pilot wire) which is used to monitor continuity of the ground wire. In most jurisdictions, continuity of the ground conductor in a trailing cable must be monitored in order to ensure that, in the event of a ground fault, a predictable ground-fault current will flow and thereby allow coordinated ground-fault protection to be reliably achieved.,
At the junction box of the equipment receiving power from a trailing cable, a termination device is connected between the ground-check wire and the ground wire. At the other end of the trailing cable (i.e. the location of the power source) the ground-check and ground wires are connected to terminals of a ground-check monitor which measures and monitors the electrical characteristics between these terminals. The termination device used with a given ground-check monitor is matched, by its circuit characteristics, to that particular monitor type. In operation, the ground-check monitor permits the trailing cable to be energized (i.e. powered) only if the circuit characteristics associated with the termination device are detected; otherwise, if either the ground wire or the ground-check wire is broken, if the ground-check wire is shorted to ground, or if the termination device is removed, the ground-check monitor will not detect such characteristics and will cause the cable to be de-energized. Typically the termination device is a diode or zener diode.
In some applications, power is supplied to two pieces of equipment from one remote power supply. It is desirable to use one long trailing cable to feed a splitter box which feeds two separate cables to supply power to the two pieces of equipment (loads). This configuration of the trailing cable is sometimes referred to as a Y-configuration (or T-configuration). However, if a termination device associated with the ground-check monitor is connected across the ground-check and ground wires at the end of each cable connecting the loads to the power source (i.e. so as to monitor the ground wire of each arm) a problem will be created. In such a configuration, the electrical characteristics produced by each of the parallel termination devices corresponds to the normal characteristics of the termination device which the monitor is designed to detect and, therefore, the ground-check monitor would detect such characteristic and fail to cause power to be removed if either termination device is removed.
The problem of how to monitor the ground wires of all sections of such a branched trailing cable has to date been addressed by expensive and complicated circuit installations. Such installations utilize either a second ground-check monitor at the splitter box to monitor the ground-check wire in the circuit of the second load or a multiplexer at the splitter box to switch the monitor between the ground-check wires of each of the two load circuits. Therefore, there is a need in the industry for a solution to this problem which is economical yet reliable from a safety perspective.